1. Field of the Invention
The invention relates to valve gears for load-change valves of four-stroke internal combustion engines, in which optional opening of the load-change valve during a different work cycle can take place in addition to opening in accordance with the cycle.
2. The Prior Art
A valve gear of this type, for outlet valves, is described in PCT Application Publication No. WO 03/067067, FIG. 2 and FIG. 4. Outlet valves held in the closed position by spring force are opened according to the cycle, in usual manner, by an outlet cam, by way of a toggle or pivot lever, as well as a pressure plate that acts on the stems of the valves, so that the waste gases can flow out of the cylinder chamber into the waste gas tract via the outlet channel at the end of the expansion cycle and during the movement of the piston to the upper dead point. An intermediate element is articulated onto the toggle or pivot lever, so as to pivot. This element is firmly supported against the toggle or pivot lever by a hydraulic pressure cylinder to which pressure can optionally be applied or, if no pressure is applied to the pressure cylinder, can rotate freely about its articulation point. When pressure is applied to the hydraulic pressure cylinder, the outlet cam stands in engagement with the toggle or pivot lever during the intake process, by way of the intermediate element. This lever is moved exclusively by the highest region of the lifting curve of the outlet cam. The outlet valve is therefore additionally opened during the intake process, but this happens with a significantly lesser time cross-section than during expulsion of waste gas. This is shown in FIG. 1 of the PCT application mentioned above.
By means of the additional opening of the outlet valve during the intake process, waste gas gets into the cylinder from the waste gas tract, by way of the outlet channel, and subsequently mixes with air that is drawn in, or with the fuel/air mixture that is supplied, during intake and compression. In order to achieve a slight time cross-section for returning waste gas during the intake cycle of waste gas, only the highest region of the lifting curve of the outlet cam is utilized. This highest region has a lifting characteristic that is configured to be optimal for the return of waste gas. Such a lifting characteristic forms a compromise with regard to an optimal lifting characteristic for the outlet process of waste gases.
It is also disadvantageous that the intermediate element articulated onto the toggle or pivot lever is moved along during all outlet valve movements, i.e., also during the outlet process of the waste gases, without this being functionally necessary.